THE NATURE, TESTING, AND VARIABLES INFLUENCING
FIXATION DISPARITY; ROLE OF THE FUSION LOCK
1 by Leland W. Carr with Dr. James J. Saladin
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I Introduction
A fixation disparity is a small angular measurement of the mis alignment of the two eyes which can occur while still permitting single, fused binocular vision. It represents a small error in the aiming of the eyes which occurs without diplopia being detected.
Fixation disparity is allowed because of the slight "slippage" pro vided to the fusional system through the existence of Panum's fusional areas. So long as binocular alignment is precise enough to place the two retinal images of a single object within corre sponding Panum's areas, the final perception is likely to be single and fused. Sensory fusion thus occurs in spite of a small error in motor fusion.
A slight muscle imbalance is the rule, rather than the excep tion in individuals even with normal assymptomatic binocular systems.
It is rare that all twelve extraocular muscles are precisely bal anced in their agonist-antagonist relationships, and thus the inate drive to achieve single vision requires a fusional effort to over come existing imbalance. When binocularity is dissociated (as with a covertest) the fusional drive is interrupted and the eyes deviate out of alignment under the influence of the muscle imbalance. This deviation under dissociation is referred to as the heterophoria.
When both eyes are permitted to view without dissociative conditions
the fusional drive to achieve single binocular vision pulls the eyes
toward alignment in opposition to the phoric "stress" operating to
deviate the alignment.
Because of Panum's areas, motor fusion need only achieve
I 2 alignment within the corresponding regions, rather than obtain pin point- precise binocular alignment, and fusion will occur. Thus
fixation disparity measures the amount of misalignment occurring under fused (though "stressed") binocular viewing situations.
It is the intent of this paper to briefly summarize the his tory and techniques of measuring fixation disparity, and to then examine one of the major variables of the measurement technique - the size of the fusional stimulus (fusional "clue", fusional "lock", fusional "ring") and its impact on the measurements obtained.
History
The earliest work on fixation disparity came from observations by Hofmann and Bielschowsky with the synoptophore which revealed that exo f.d. increased as convergence demand increased.
Ames and Glidden also worked with haploscopic targetsJ consist ing of central large block E l e tters with peripheral horizontal and vertical dots. They found that superimposion of the peripheral dots could still leave the E slightly doubled indicating a slight mis alignment of the lines of sight, which they called an "inexactness of fixation associated with a heterophoria."
The majority of the f.d. experimentation was done under the supervision of Ogle. (I) His experiments show that patients with a phoria (in which the eyes have a tendency to turn inwards or out wards) will actually under - or overconverge by a small amount, thus resulting in disparate images falling on the two retinas.
These disparate images will be fused, however, and the subject will not be aware of disparity.
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Through trial-and-error experimentation Ogle found that ver tical vernier lines would provide the most accurate means of investi gating these small deviations in alignment of the eyes.
To measure fixation disparity most accurately requires that the vernier lines be seen foveally where accuity is at its finest, and that they be seen bi-ocularly with one eye viewing one line while the other eye views the other line. This means that central region fusional details must be eliminated, and thus periphera~ fusion clues are utilized while central target lines are used to measure misalignment under fusing conditions.
In a patient with normal ocuities in both eyes, a displace ment of + 1.0 minute of arc can be detected in vernier lines.
An interesting sidelite is that the test of fixation disparity using small centrally located vernier lines can indicate a foveal suppression if one line is not visible while all of the peripheral clues, and the other line, are visible.
Ogle determined that the fixation disparity was evidence that an oculomotor muscle imbalance exerts a continuous effort to devi- ate the eyes to the heterophoric position, and that it was an entity called "fusional convergence" which acted in opposition to the phoria to direct the eyes to "near-enough" alignment that diplopia was avoided. A small error in precise convergence (or divergence) occurs most often in the same direction as the phoria, thus causing a sin gle fixation point to be imaged onto slightly noncorresponding (dis parate) points on the two retinas; the amount of this disparity
Ogle called the "fixation disparity". A f.d. cannot be larger than the limits of corresponding Panum's fusional areas or diplopia (or
/ 4 suppression) will occur.
Therefore, Ogle tells us that a fixation disparity is evidence that fusion occurs without full compensation of the phoria, and
~;;. represents a manifest deviation of the visual axis in spite of sue- cessful fusion. The size of the fixation disparity depends pri- marily upon the size of the muscle imbalance (the amount of the phoria), but Ogle also felt that it must depend upon the stimulus
to fusion.
According to Ogle, as ·larger fusional clues stimulate more peripheral areas of the retina where Panum ' s areas are larger, he would expect fusion to occur with less precise binocular alignment.
With more "slop" permitted in alignment, he should expect to have
larger fixation disparities.
Ogle also felt that the fixation disparity will increase as
the effort to maintain fusion is incr eased. As one works harder
to achieve fusion (for examp l e near t he vergence limits) the f.d.
will increase in size .
Clinical Measurement of Fixation Disparity
Measurement of fixation disparity becomes a more meaningful
indicator of a healthy binocular system when it is checked under
various levels of fusional "stress". Prisms are used to alter the
demand on the convergence/divergence mechanism, shifting the fusional
demand, and changing the innervation to the extraocular muscles.
Fixation disparity is measured under various conditions of fusional
stress. (II)
Base-in prism will create an innervational esophoria because
the stimulus to accommodation will be nearer than the stimulus to
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fusion, requiring negative fusional innervation in order to see clearly and singly. Accommodative convergence is acting to turn
the eyes inward while fusional drive acts to aim the eyes outward
(innervational esophoria!)
Base-out prism acts in a reverse manner to create the inner vational situation of an exophoria.
As base- in prism is increased toward the limits of the nega tive vergence, then the fixation disparity will most often increase in the eso disparity direction (an underdivergence), showing an increasing esophoric oculomotor imbalance. Once the base-in break point is reached diplopia occurs and the eyes shift to their natural phoric position.
Again the reverse situation is found for increasing base- out prism. Host often this will give an increasing exo-disparity up to the convergence breakpoint .
Fixation disparities are quite s mall angles, measured in minutes of arc. They are interpreted based upon their curve when plotted on a two coordinate graph. The abscissa lists the base-in and base- out prism power placed before the eyes, while the ordinate lists the eso or exo disparity measured.
That value of prism power before· the eyes when the f. d. is found to be zero is called the "associated horizontal phoria". It indicates the amount of muscle imbalance when the eyes are precisely aligned upon the fixation target (notice the assumption of normal retinal correspondence has been made!) The term "associated" phoria is used because the binocularity has not been drastically ''disas sociated" as by maddox rod or vertical prism used to determine the
I 6
"disassociated phoria". The point on the fixation disparity graph where the oculomotor imbalance is zero (where the curve crosses the abscissa) indicates the prism value of the associated phoria. It indicates the relative rest position of the ext~aocular muscles for the subject for that observation distance; when fusion is main- tained. (III)
Accuracy of f.d. measurement by the use of a two-line nonius alignment is acuity dependant, and subjects with 20/20 acuity in each eye are able to discriminate 1' of arc or even smaller dis parities. Sensitivity is altered only slightly if both eyes are equally blurred, but is more significantly decreased if the two acuities are significantly different, and is quite variable in sub jects with larger oculomotor imbalances. The amount of f.d. tends to vary directly as the size of the phoria.
As mentioned previously, Ogle considered the role of "periph eralness" of the fusion-lock in determinations of f.d. by the ver nier lines method. He expected more retinal slippage (more misalign ment) to be tolerated by the system before diplopia occurred, when using a more peripheral fusion target. The small Panum's areas associated with the macular area could not tolerate much disparity, or misalignment, before diplopia would occur. Thus Ogle predicted larger f.d. for a more peripheral target for fusion.
Ogle's preliminary investigations did find larger fixation dis parities when the fusion-lock portion of the test target was made larger (therefore made more peripheral on the retina). One major finding of this initial work was that the associated phoria was not changed by the size of the fusion-lock. Instead, the graphed curves
I 7 while varying in slope and amount all seemed to cross the abscissa at the same point - to pivot around the value of the associated phoria. (IV) However later experiments by Ogle, as well as by
Shepherd gave conflicting results which seemed to show the f.d. was actually not affected by the size (peripheralness) of the tar get's "frame for fusion", (fusion lock). Shepherd concluded that f.d. was not dependant on Panum's area in the periphery. (V)
There are thus a number of interrelated variables operating to influence the fixation disparity curve. We have mentioned spe cifically the impact of the phoria, the muscle imbalance, the sta bility of the vergence system, the individual visual qcuities, and the possible role of the size of the fusion-lock. One would also predict that accommodation through accommodative-convergence can contribute toward, or help relieve, stress on the fusional system.
Thus fixation distance and ophthalmic lenses may contribute to the fixation disparity curve through their impact upon accommodative demands.
The goal of this study was to look at the impact of the size and peripheralness of the fusion-lock upon the f.d. plot. Recog- nizing the existence of the several other impacting variables, we determined to evaluate our data in terms of several curves plotted for each of twenty subjects. Each individual curve measured with a different fusion lock can be usefully assessed only in compari son to other individual curves generated for that same subject. In this way we can judge the role of the fusion lock alone, by assum ing that all other variables remain constant for that individual subject. For instance the subject with .50 anisometropia will 8 demonstrate the aniso under all viewing situations; the accommoda
tive-excess subject will demonstrate the disorder as a constant while generating each curve. In our experimental design the only variable was to be the size of the peripheral fusion lock.
After determining the fixation disparity data for each subject under the different fusion-stimulus conditions, the data would be combined to create a set of fixation disparity curves for a "theo retical observor" made from the mean values obtained from the com- bined subjects. In this way the averaged data would minimize the individuals' variability by spreading it across the several plots and averaging it into a mean value.
The "theoretical observor" would produce a series of fixation disparity curves for each of the different sized fusion lock tar gets. These mean plots would then be combined, averaged, and a graph consisting of the mean of the mean values generated. This curve would serve to describe the standard deviation and allow sta tistical analysis of the impact on fixation disparity caused by changing the size of the fusional stimulus clue, while minimizing
the role of individual variables other than fusion lock size.
Experimental Design
Twenty volunteer observors were chosen based on their willing ness to participate in the study. They were all in the 20 to mid dle 30 age group, with approximately one half not associated with the college.
Each subject was seated behind a phoropter to allow use of binocular Risley prisms, and each wore a pair of research grade polarized spectacles precisely aligned to obtain hi-ocular percep-
I 9 tion of the polarized vernier target lines and binocular percep tion of the fluorescent peripheral fusion ring. Subjects made all observations wearing their habitual distance prescription in the form of their own contact lens or spectacle correction, behind the polaroids.
The subject controlled the vernier line adjustment by means of a knob which was geared . to the upper line segment in such a way as to displace it horizontally as the knob was twisted, until the perception of alignment resulted. This adjustable upper line was seen by the left eye only, due to polarization. The adjustment knob was at the same time connected to a volt meter calibrated to read the actual disparity of the vernier lines in terms of minutes of arc.
The bottom line was seen by the right eye only, and was con tinuously flashed to overcome any macular suppression tendencies.
Additionally the right eye viewed two short, narrow polarized hori zontal lines placed below the vertical nonius line, and intended to help stabilize accommodation by requiring accurate focusing in order to maintain resolution of their separateness.
Fusion ring stimuli were prepared with fluorescent green paint on flat black backgrounds. The center portion of each ring was r emoved to allow visibility of the vernier alignment apparatus with in the ring when the fusion target was placed in front of the ver nier portion of the apparatus. A horizontal fluorescent line bisected each fusion ring and separated the vernier lines. This was done to act as a strong stimulus for precise vertical fusion to minimize the influence of vertical disparities.
I 10
The experiment was performed in total darkness except for Bur
ton-lamp blacklight illumination of the fusion ring and fluorescent
horizontal line, and the polarized light emerging from the vertical vernier lines and the horizontal accommodation lock. Peripheral
extraneous fusional clues were eliminated by the flat black paint
on the testing apparatus and the total darkness of the room.
The testing apparatus was located at one meter before the
phoropter. Initial alignment adjustments were made with no fusional
ring stimulus, followed in turn by four progressively larger ring
shaped fusional locks. For each fusional situation the fixation
disparity was measured for prism of 0, 4 base-in, 4 base-out, 8
base-in, 8 base-out, 12 base-in, 12 base-out, and 20 base-out prism
diopters, respectively. In this way five separate fixation dis
parity curves were generated for each of the twenty subjects, while
attempting to minimize tonic and innervational changes in the ver
gence system.
Each value of fixation disparity was recorded and graphed
under the appropriate size fusional clue.
Upon completion of all twenty test sessions, the mean fixa
tion disparity was found for each value of prism, for each of the
five fusional situations. In this way the plots for the "theoreti
cal observer" were generated for each fusional situation. Instances
of diplopia were dropped from the mean calculations except for
those prism demand situations where diplopia was the common response,
in which case "diplopia" was accepted as the mean value.
Finally, from this set of averaged data a plot consisting of
the mean-of-the means was calculated and used to determine the 11 significance of the variability for the five mean plots.
Analysis of Data
As mentioned previously, it was not felt that fixation data could be meaningful compared amongst subjects due to the large num ber of uncontrolled variables. However the data for each single observer could be compared to other data for that same observer under conditions where only the size of the fusion ring was varied.
By looking at variability among the subgroups of data for a single subject, the key variable becomes the size of the fusion lock. A statistical analysis of variance, as well as simple "eyeball" com parison of the subgroups of data, allows evaluation of impact of differential fusion clues for that observer. First the graphs were objectively compared and a "common sense" evaluation made of the significance of the fusion lock. Next the Fisher or "F" test was used to statistically analyze the variance among the subgroups.
The "F" test assesses the effect of different treatment upon sub group means. With five subgroups of data based on the five dif ferent fusional conditions tested, the mean for each group was com pared to the other four subgroup means for a significant amount of difference. Thus the effect of altering the fusional lock was evaluated for each individual observer. Similarly the averaged data (our so-called "theoretical observer") was evaluated by obser vation, then by the F test.
In all twenty sets of data for individuals, as well as the data for the theoretical observer, the size of the fusion ring was found to be insignificant. The graphs appeared similar with allow ance for chance variability. This was confirmed in all cases by the F test where the differential treatments were found insignifi-
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cant at the 5% and 1% levels. The size of the fusion ring did not
change the disparity curve. Peripheralness of the fusion lock did not change the curve found.
Summary
By testing the fixation disparity while varying the size and
peripheralness of the clue for fusion, the impact upon the fixation disparity plot was determined. Direct observation and statistical analysis for twenty observers showed no significant impact of the
fusion lock upon the fixation disparity measured. For these twenty subjects it was not how they were influenced toward fusion, but the
fact that their systems were driven to seek fusion in spite of prism-induced obstacles that resulted in nearly identical perfor mance under the five situations tested. The fusion lock was of les-
ser importance. Sources
I. Ogle KN Martens TG, Dyer JA: Oculomotor Imbalance In Binoc ular Vision And Fixation Disparity Philadelphia: Lea And Febiger, 1967.
II. Sheedy JE: Actual Measurement Of Fixation Disparity And Its Use In Diagnosis And Treatment. JAm Optom Assoc 51(12):1079- 1084 Dec. 1980.
III. Griffin JR: Binocular Anomalies And Procedures For Vision Therapy Chicago: Professional Press, 1976.
IV. Ogle
V. Shephard JS: A Study Of The Relationship Between Fixation Dis parity And Target Size. Am. J. Optom. 28 391-404 1951.
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